"frama-c -wp -wp-rte" fails to verify this rather tautological spec because the RTE pass inserts the assertion

/*@ assert rte: shift: 0 ≤ arg; */

I think this is an incorrect reading of the ISO C 90 and C 99 specs: right-shift of a signed integer expression that has a negative value is implementation-defined, but is not an undefined behavior. (See ISO C 99 section 6.5.7 item 5.)

I would suggest that WP and RTE accept the spec above, so that I can actually reason about the right shift, probably by adding axioms specifying how right shifts of negative numbers work on the platforms I am (and everyone is) interested in.

A new kernel option (-warn-right-shift-negative) has been introduced for controlling this behaviour. It is deactivated by default (while -warn-left-shift-negative is activated by default), hence no assertion is now emitted by default on your example.

This option is supported by plug-ins Eva and RTE. The assertions generated by RTE impact plug-ins E-ACSL and WP.